1. Technical Field
The present invention relates generally to backlight modules, and more particularly to a backlight module for use in a direct type liquid crystal display.
2. Background
Liquid crystal displays (LCDs) are widely used in various electronic products such as personal digital assistants (PDAs), notebook computers, digital cameras, digital camcorders, mobile telephones, computer monitors, liquid crystal televisions, and the like. Since LCD panels used in LCDs are not self-illuminating, they need a backlight module with a light source to illuminate the panels.
The backlight module enables colors of an image created on the LCD panel to be reproduced in real color by emitting white light from the rear of the LCD panel. As for the light source of the backlight, in the early days, Cold Cathode Fluorescent Lamps (CCFL) or External Electrode Fluorescent Lamps (EEFL) were used. However, with the advent of efficient Light-emitting Diodes (LEDs) with good physical and chemical properties, it has been suggested that LEDs may be employed as backlight light sources.
LEDs are semiconductor light-emitting devices, which emit various colors of light. LEDs can be composed of various semiconductor materials, such as GaAs, AlGaAs, GaN, InGaN and AlGaInP (Where Ga is gallium, As is arsenic, Al is aluminium, N is nitrogen, In is indium and P is potassium). Compared to other sources of illumination LEDs have the advantage of having a low operation voltage, high brightness, short response time and long lifetime. Furthermore, an important advantage of LEDs is that they are free of mercury, and are thus more environmentally friendly than other light sources, such as CCFLs.
Generally, an LED includes a base, a cover made of a plastic material, and an LED chip mounted in the cover. The cover has a top surface for emitting light generated by the LED chip.
As for the backlight module employing the LED, a conventional direct illumination type backlight module in which the light source of the backlight module emits light directly under the light guide plate of the backlight module panel is proposed. Due to the point light source, the illumination on the incidence surface of the light guide plate has poor uniformity. A number of darker areas are distributed on the incidence surface opposite to the central point between two adjacent LEDs. This will cause poor illumination uniformity of the light emitted from the emissive surface. Therefore, a backlight module employing LEDs as the light source with high brightness uniformity is desired.